The present invention relates to a high-hardness prehardened steel for cold working which is used for producing dies etc. for the blanking, bending, drawing and trimming of steel sheets used in automobiles, home electric appliances, printed circuit boards, agricultural implements, etc., a die made of this steel, and a method of working this steel which contributes to the achievement of these means.
Working such as blanking is used in the manufacture of parts of automobiles, home electric appliances etc. Die materials used in dies and, in particular, die materials for cold working contain large amounts of carbides in order to provide wear resistance and, Further, materials which have high chromium contents are required to ensure excellent hardenability and toughness. For example, high-C and high-Cr steels such as JIS-SKD11, which is an alloy tool steel specified in JIS G4404, are used for this purpose. When wear resistance is not particularly required, low-alloy steels such as JIS-SKS3 are also used in a hardened, tempered state by conducting tempering at a temperature of not more than 300xc2x0 C. after oil hardening.
In recent years, however, automakers etc. have carried out cost reductions in every field in order to beat their price competition and it has become necessary to reduce die-fabrication man-hours in the working related to the fabrication of dies. On the other hand, as a general social trend, there is a shift to small-amount-and-many kinds production and, from this viewpoint, a matter of concern has been placed on how rapidly dies can be fabricated.
In the present usual fabrication of dies, materials in an annealed state are subjected first to the rough working of a shape mainly by cutting work and then to the hardening and tempering treatment in order to increase their hardness to a level necessary for the working of products such as blanking. When the hardening and tempering treatment is performed, changes in size and deformation occur due to the heat treatment. Therefore, the fabrication of the dies is completed by performing finish working such as cutting and grinding. For this reason, in order to improve the die-fabrication efficiency, it is desirable that machinability in an annealed state be good and simultaneously that changes in size and deformation due to heat treatment be small, and a tool steel for cold working as described in JP-A-11-92871 is proposed.
However, since further reduction in both of man-hours for the die fabrication and the cost of the die fabrication is required at present, the need for so-called xe2x80x9cprehardened steelsxe2x80x9d is increasing. In the prehardened steels, the die-fabrication efficiency can be increased because the cutting work is performed in a hardened and tempered state of the steel while omitting the heat treatment and finish working both having been usually performed after the cutting work.
At present, die fabrication by use of the prehardened steel is performed in the fields of a part of plastic dies and a hot-forging die etc. However, the hardness of such dies is about 40 HRC and does not reach hardness a level not less than 50 HRC which is required in blanking etc. by cold working. This is because the higher the hardness thereof is, the higher both of the resistance and impact applied to tools becomes during cutting, and these resistance and impact exceed the strength of the tools, with the result that the early breaks thereof occur, reaching tool life.
On the other hand, in the field of the cutting work the improvement in the cutting efficiency has advanced because of high-speed cutting. However, in the high-speed cutting of high-hardness materials exceeding 50 HRC, the softening of tools or the fusing-and-adhering of worked material onto the tools is accelerated by an excessive rise in the cutting temperature in addition to the above impact to the tools, so that the premature termination of the tool life also occurs. In a conventional tool steel for cold working, that is, JIS-SKD11, there are many cases where it becomes impossible, for the reasons disclosed above, for this tool steel to satisfy the requirements for the die-fabrication efficiency and tool life which requirements arise from the standpoint of the shortening of man-hours of die fabrication, insofar as the cutting work is concerned which is performed in the high-hardness state achieved after the hardening and tempering.
Further, the steel of JIS-SKS3 does not always meet the requirement for an improvement in the die-fabrication efficiency although its machinability in the hardened and tempered state is good in comparison with the steel of JIS-SKD11. In addition, because this steel is an oil-hardening and low-temperature tempered steel, it poses the problem of the occurrence of strain during electric discharge machining which is often used in die fabrication instead of the cutting work.
As explained above, in the tool steels conventionally used for producing dies etc., it is practically difficult to perform the cutting work in the prehardened state of the tool steels with the hardness level not less than 50 HRC.
The object of the invention is to obtain a tool steel having an improved machinability after hardening and tempering so that working in the prehardened state thereof may become possible, a die made of this steel, and a method of working this steel which contributes to the achievement of these means.
After the present inventors have performed intensive research regarding the chemical compositions of steel which bring about both of a high hardness level not less than 50 HRC and good machinability at this high hardness level, they have found an appropriate composition balance and have achieved the present invention.
According to the first aspect of the invention, there is provided a high-hardness prehardened steel for cold working, which contains, by mass, not less than 0.3% but less than 0.5% C, 0.7 to 2.0% Si, and 0.08 to 0.25% S, the steel being hardened and tempered to a hardness not less than 50 HRC and preferably to a hardness not less than 55 HRC.
According to the second aspect of the invention, there is provided a high-hardness prehardened steel for cold working, which consists, as its specific chemical composition, by mass, not less than 0.3% but less than 0.5% C, 0.7 to 2.0% Si, 0.1 to 2.0% Mn, 0.08 to 0.25% S, 0.5 to 15.0% Cr, at least one selected from the group consisting of W and Mo the total content of which at least one is not more than 3.5% in terms of (Mo+1/2W), not more than 4.0% V, not more than 0.15% N, and the balance Fe and incidental impurities.
According to the third aspect of the invention, there is provided a high-hardness prehardened steel for cold working, which consists, by mass, of 0.3% to 0.45% C, 0.8 to 2.0% Si, 0.1 to 2.0% Mn, 0.08 to 0.25% S, 4.0 to 6.0% Cr, at least one selected from the group consisting of W and Mo the total content of which at least one is not more than 2.0% in terms of (Mo+1/2W), not more than 1.0% V, not more than 0.15% N, and the balance Fe and incidental impurities.
According to the fourth aspect of the invention, there is provided a high-hardness prehardened steel for cold working, which contains in addition to the above components: at least one selected from the group consisting of Nb, Ta and Ti the total contents of which at least one is not more than 0.4% by mass; and at least one selected from the group consisting, by mass, of not more than 4.0% Ni, not more than 2.0% Cu, not more than 5.0% Co, not more than 0.2% Zr, not more than 0.15% Se, not more than 100 ppm Ca, and not more than 1.5% Al.
According to the fifth aspect of the invention, there is provided a high-hardness prehardened steel for cold working, which is used after being subjected to cutting work in the above hardened-and-tempered state and in which the speed of the cutting is not less than 50 m/min.
According to the sixth aspect of the invention, there is provided a die for cold working which is fabricated by cutting the high-hardness prehardened steel for cold working.
The attainment of both of the prehardened steel and the die for cold working of the invention is also greatly attributed to the establishment of optimum conditions for prolonging the service life of cutting tools in the cutting of the steel hardened and tempered to the high hardness. Namely, according to the seventh aspect of the invention, there is provided a method of performing cutting work of a steel hardened and tempered to a hardness not less than 50 HRC which cutting work is performed at a cutting speed not less than 50 m/min, the steel to be worked containing, by mass, not less than 0.3% but less than 0.5% C, 0.7 to 2.0% Si, and 0.08 to 0.25% S.